Charlestown Rt 12 Repair Update … November 18, 2021

Dear Elected Officials for Charlestown, Walpole and Claremont:

A total of 4 bids were received on the Charlestown Rt 12 Repair project by today’s deadline of November 18, 2021 @ 2:00 pm.  The Department is reviewing the bids and will select a contractor shortly.  Due to the time required to draft the contract and other documents for approval, the Department will request authorization to award the contract at the November 22, 2021 G&C meeting, and subsequently execute the contract with the selected contractor.

Pending the review of bids, selection of a contractor, and G&C approvals, it may be helpful to understand the work that is required due to the site constraints and resulting complexities.

The total project length is 700 feet and begins with construction of the 400 foot soil nail wall. Once selected and approved, the contractor will seek input from the soil nail expert. The soil nail wall is required to stabilize the soil slope and avoid soil movement (i.e. landslide), which could impact the active rail line.  The soil nail wall will be constructed from top to bottom in three separate phases or rows, with each row being approximately 3 feet deep. The total vertical excavation will be approximately 10 feet deep.  Construction of the top row of the soil nail wall involves excavating the top 3 feet of soil along the entire length of the proposed wall. Then holes are drilled horizontally 3 feet apart into the exposed vertical soil surface.  Steel bars (“nails”) are inserted into the drilled holes and grouted. Tension is applied to the steel bars, ensuring that the grout adheres to the steel bars. Then shotcrete (wet concrete applied with a hose) is applied to the vertical soil surface.  Finally, the steel bars are anchored in place with large bolts. This process is performed twice more in the same sequence, going down another 3 feet for the installation of each row.

Curing the grout and the shotcrete is the major factor impacting construction time.  Samples of the grout and shotcrete will be tested.  When the test results show a certain strength (2000 psi) for the first constructed row, excavation can begin on the second row; the same testing process is used before constructing the third and final row.  Temperature is also the major variable in curing times, requiring blankets or tents for cold weather.

It takes an estimated 3 weeks to complete the construction of each row.  Based upon this estimation, construction of the entire wall will take approximately 9 weeks to complete.  Once the wall is constructed, the roadway can be rebuilt.

This situation on Rt 12 occurred due to heavy rains that undermined the road.  In order to avoid a reoccurrence of this situation, the plans require the contractor to create a drainage system to limit the groundwater and collect stormwater.  A “drainage blanket” will be constructed with stone to create a path for groundwater to flow.  The roadway will be strengthened with 7 feet of structural fill. Then a foot of crushed stone will be laid as base material before 5 inches of pavement is applied.

I hope this explanation is helpful in understanding the work to be performed.  I’ve also attached a page of graphics that show the construction process for the soil nail wall.

Next week I plan to update you on the contractor selection.  As construction begins, I will include photos of the work in progress. Please let me know if any other information would be useful.

My Best,

Kathy

Kathleen Mulcahey-Hampson

Legislative Liaison